The brief
The C180 had started squeaking under braking, the high-pitched chirp that means the rear pads have worn down to their wear indicator. He brought it in, which is the right call, that squeal is the warning that comes before the grind. Brake pads are a wear item, designed to be used up and replaced. As they thin, a little metal tab on the pad starts to touch the disc and squeals to tell you it's time, before the friction material runs out entirely and the backing plate starts grinding the disc. The C180's rear pads had reached that point. Caught at the squeal stage it's a straightforward pad change and the discs are usually still fine, so they needed doing before it went further.
The diagnosis
On the lift the rear brakes confirmed it, the pads worn down to the wear marker with little material left, but the discs measured up still within thickness and ran true, no scoring, caught in time. The calipers and slides were freed off and checked and they were fine. The fronts still had plenty of life. So it was a rear pad replacement, a fresh pair, with the discs cleaned up and kept, because they were still good and there was no reason to change a serviceable disc.
The work
The worn rear pads were removed, the calipers and slide pins cleaned and greased so they move freely, the discs cleaned up, and a new genuine Mercedes-spec set of rear pads fitted, with the electronic parking brake retracted and reset the proper way for the job. The pads were bedded in so they'd grip evenly from the start. A road test confirmed quiet, even, progressive rear braking with a firm pedal and no pulling.
The outcome
Quiet brakes, a firm pedal, even bite front and rear, and the squeal gone, with the discs still serviceable for plenty more. The C180 went home stopping properly again. Catching pads at the squeal stage keeps it to a simple pad change instead of pads and discs later, so doing the rear pair when the wear marker called put the braking right at the cheap end of the job.